A radio communication system is comprised, at minimum, of a transmitter and a receiver. The transmitter and the receiver are interconnected by a radio-frequency channel to permit transmission of an information signal therebetween.
Typically, the information signal is impressed upon a radio-frequency electromagnetic wave by a process referred to as modulation to permit transmission of the information signal between the transmitter and the receiver. The radio-frequency electromagnetic wave is referred to as a carrier wave which is of a particular frequency, and the carrier wave, once modulated by the information signal, is referred to as a modulated information signal. The modulated information signal may be transmitted through free space to transmit thereby the information between the transmitter and the receiver.
Various modulation techniques have been developed to modulate the information signal upon the electromagnetic wave. Amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), and composite modulation (CM) are four of such modulation techniques. Frequency and phase modulation techniques are collectively known as angle modulation.
In general, an amplitude modulated signal is formed by impressing (i.e., modulating) an information signal upon a carrier wave such that the information signal modifies the amplitude of the carrier wave corresponding to the value of the information signal.
An angle modulated signal formed is formed by impressing (i.e., modulating) an information signal upon a carrier wave such that the information signal modifies the phase (or the time differential of phase, frequency) of the carrier wave corresponding to the value of the information signal. Angle modulation does not cause the amplitude of the carrier wave to vary, and the information content of the modulated information signal is contained in the variation of the phase (or frequency) of the signal. Because the amplitude of an angle modulated signal does not vary, an angle modulated signal is referred to as a constant envelope signal.
A composite modulated signal is formed by impressing (i.e., modulating) an information signal upon a carrier wave such that the information signal modifies both the amplitude and the phase of the carrier wave. Conventionally, in order to form the composite modulated signal, the carrier wave (or a carrier intermediate frequency, i.e., IF, source) is first separated into sine wave and cosine wave carrier portions. Separate portions, referred to as the in-phase (or I) and the quadrature (or Q) components, of the information signal are impressed upon the cosine wave and sine wave carrier portions of the carrier wave, respectively. The sine wave and cosine wave components are then recombined, and the resultant signal, the composite modulated signal, varies in both amplitude, and, additionally, phase. Composite modulation is advantageous in that a composite modulated signal permits a greater amount of information to be transmitted within a frequency bandwidth than a signal generated by any of the previously mentioned modulation techniques. See, for instance, a discussion in the text Introduction to Communication Systems, 2nd Ed, by Ferrel G. Stremmler, ISBN 0-201-07251-3, pages 590-596.
A family type of composite modulation is quadrature amplitude modulation (QAM). In this modulation method, as conventionally applied to a binary information source, the binary data stream is separated into bit pairs. The individual bits of the bit pairs are converted from unipolar to bipolar format, passed through a pair of electric wave filters, and applied to the multiplier pair whose other inputs are the sine and cosine components of the carrier or carrier IF signal. A particular type of QAM is .pi./4-shift DQPSK (for differential quadrature phase shift keying), in which the input data stream is encoded so that the composite modulated carrier shifts in increments of .+-..pi./4 or .+-.3.pi./4 according to the input bit pairs. This modulation method, conventionally implemented, is discussed in Digital Communications, by John G. Proakis, 1st Ed., ISBN 0-07-050927-1, pages 171-178.
A receiver which receives a modulated information signal, such as a one formed by one of the above described modulation techniques, includes circuitry to detect, or otherwise to recreate, the information signal modulated upon the carrier wave. This process is referred to as demodulation. As many different modulated information signals may be simultaneously transmitted by a plurality of transmitters at a plurality of different frequencies, a receiver contains tuning circuitry to demodulate only those signals received by the receiver which are of certain desired frequencies. The broad range of frequencies at which modulated information signals may be transmitted is referred to as the electromagnetic frequency spectrum. Regulation of radio-frequency communications in certain frequency bands of the electromagnetic frequency spectrum minimizes interference between simultaneously transmitted signals.
For example, portions of a 100 MHz band of the electromagnetic frequency spectrum (extending between 800 MHz and 900 MHz) are allocated for radiotelephone communication, such as, for example, communication effectuated by radiotelephones utilized in a cellular, communication system. Existing radiotelephones contain circuitry both to generate and to receive radio-frequency modulated information signals.
A cellular communications system is created by positioning numerous base stations at specific locations throughout a geographical area. Each of the base stations is constructed to receive and to transmit modulated information signals simultaneously to and from radiotelephones to permit two-way communication there between. Each of the base stations is provided with means to communicate with one or more switching offices which permit connection to the conventional telephone network.
The base stations are positioned at locations such that a radiotelephone at any location throughout the geographical area is within the reception range of at least one of the base station receivers. The geographical area is divided into portions, and one base station is positioned in each portion. Each portion of the geographical area defined thereby is referred to as a "cell".
Although numerous modulated information signals may be simultaneously transmitted at different transmission frequencies, each modulated information signal, when transmitted, occupies a finite portion of the frequency band. Substantial overlapping of simultaneously transmitted modulated information signals at the same frequency in the same geographic area is impermissible as interference between overlapping signals at the same frequency could prevent detection of either of the transmitted modulated information signals by a receiver. Frequency re-use is permitted if sufficient geographic separation exists between base sites using the same frequency, because of the attenuation of signals with distance.
To prevent such overlapping, the frequency band allocated for radiotelephone communication in the U.S. is divided into channels, each of which is of a 30 KHz bandwidth. A first portion, extending between 824 MHz and 894 MHz of the frequency band, is allocated for the transmission of modulated information signals from a radiotelephone to a base station. A second portion, extending between 869 MHz and 894 MHz of the frequency band is allocated for the transmission of modulation information signals from a base station to a radiotelephone.
Increased usage of cellular communication systems has resulted, in many instances, however, in the full utilization of every available transmission channel of the frequency band allocated for cellular radiotelephone communication. Other frequency bands of the electromagnetic frequency spectrum are oftentimes similarly fully utilized.
Various attempts have been made to utilize more efficiently the frequency band allocated for radiotelephone communications to increase thereby the information transmission capacity of a cellular radiotelephone communication system. Attempts have been similarly made to use more efficiently other frequency bands of the electromagnetic frequency spectrum.
Conventionally, the modulation technique utilized by radiotelephone communication systems to form the modulated information signal thereby is angle modulation. As mentioned previously, an angle modulated signal impresses an information signal upon a carrier wave to modify the frequency (FM) or phase (PM) of the carrier wave according to the value of the information signal. However, conventional angle modulation techniques use spectral resources inefficiently.
In addition to the aforementioned inefficiency of constant-envelope modulation, the voice signal to be transmitted, which contains substantial redundant information, is modulated onto the carrier without substantial removal of the redundancy. The total bandwidth required for transmission of information for a given modulation method, is directly proportional to the amount of information to be transmitted.
Thus, spectrum can be utilized more efficiently by using composite modulation. In addition, techniques have been developed to remove much of the redundancy present in the voice signal. The output of such a process is a discretely encoded data stream whose information content is low enough that it can be transmitted in bursts using the same spectrum portion required for continuous transmission of the original voice signal. This permits transmission of more than one signal at the same frequency, using the sequential time-sharing of a single channel by several radiotelephones. This technique is referred to as time-division multiple access (or TDMA).
Thus, in order to use TDMA, an information signal (such as a voice signal) which is to be transmitted is first encoded according to a redundancy-reduction scheme. Once encoded, the information signal, in encoded form, is modulated upon a carrier wave and is transmitted in sequential intermittent time segments. Other information signals may similarly be encoded, modulated, and transmitted in intermittent bursts at the same frequency by other transmitters. Thus, a greater number information signals may be transmitted within a particular frequency bandwidth. When the information signals are generated by users of radiotelephones forming a portion of a cellular communications system, a greater number of radiotelephones may be operated within a particular frequency bandwidth when such a TDMA technique is utilized.
A receiver constructed to receive a TDMA signal, such as a TDMA composite modulated signal, reconstructs the original information signal by decoding the TDMA signal transmitted to the receiver in one of the sequential time segments.
A receiver constructed to receiver TDMA composite-modulated signals may also require circuitry to perform channel equalization in the receiver. Equalizer circuitry is required to correct for delay problems associated with reflections of signals transmitted to the receiver which arrive at the receiver at different times. Because the signal received by a receiver is actually a vector sum of all signals received at a particular frequency, the signal received by a receiver may actually be comprised of the same signal received at different times as the signal may be reflected off objects prior to reception thereof by the receiver. The signal actually received by the receiver is, therefore, the sum of all signals which are transmitted to the receiver along many different paths. The path lengths may vary, and hence the signal actually received by the receiver may vary, responsive to repositioning of the receiver. Equalizer circuitry is oftentimes formed by a processor having an appropriate software process embodied therein. In order to permit optimal operation of the equalizer circuitry, the receiver should be constructed to be linear (i.e., the demodulated signals should represent accurately the original I and Q portions modulated onto the carrier).
The number of, phase of, and intensity of, signals actually received by a receiver in a multipath environment may vary over time as a result of repositioning of the receiver, or of the objects from which a transmitted signal is reflected. As a result, the phase and signal level of a received signal varies over time. This variance is referred to as "fading" of the signal. The resultant signal strength and rate of change of signal strength at the receiver is predominantly determined by how rapidly the receiver is moving through its environment, and the frequency of the channel being used. For instance, in the cellular frequency band, and when a cellular radiotelephone is positioned in a vehicle travelling at sixty miles per hour, the signal strength of the received signal can vary by approximately twenty decibels during a five millisecond period.
When two received signals of the same phase are 180.degree. out of phase, they effectively cancel each other out. The received signal's intensity approaches a null and the rate of change of the received signal intensity over time is rapid. Since the received signal strength intensity is low, the modulated information can be corrupted by noise present in the channel. A signal corrupted by noise can alter the state of the demodulated information thereby causing the receiver to receiver wrong information.
When a phase modulated signal is received, gain control circuitry should be of a design to permit rapid and continuous tracking of variations in received signal levels due to fading. In addition, a radiotelephone which generates a TDMA phase modulated signal to transmit an information signal in a cellular communication system meeting the requirements of the U.S. Digital Cellular Standard also measures intermittently the signal strengths of transmitters located in one or more cells. This process of testing signal strengths is referred to as mobile-assisted hand-off (or MAHO). The MAHO test also requires gain control circuitry which permits rapid and continuous tracking of a signal.
Optimal receiver performance is realized for composite modulation if the receiver incorporates a means for generating an estimate of the carrier phase of the received signal. Receivers which generate such an estimate are known as coherent receivers. The process of generating the phase estimate is known as carrier recovery. Several methods of carrier recovery are known.
One such method applicable to carrier recovery for receiving a signal under fading conditions in a TDMA system with .pi./4-shift DQPSK modulation is a decision feedback phase lock loop (DFPLL). A DFPLL determines what the phase-error of the received signal is relative to an ideal received signal. The phase-error signal is coupled through a loop filter to remove noise. The phase-error, with a reduced noise level, is coupled to a voltage controlled oscillator (VCO). The phase of the VCO is adjusted based on the phase-error input. The corrected phase out of the VCO is multiplied with the received signal's quadrature components to correct the phase of the received signal.
Another method applicable to carrier recovery for the aforementioned system is to raise the received signal components to the 4th power, which removes a substantial portion of the modulation, low-pass filter the resultant, and apply the low-pass filter output to a phase-correction input of the reference phase source. This can be generalized to an M-th power carrier recovery apparatus for M-ary signaling.
Another method for carrier recovery is called the generalized-Costas loop. This method requires multiplying the received signal by M-phase shifted reference signals. Where M equals 8 for .pi./4 DQPSK signaling. The reference phase signal is separated into eight components phase shifted by 0, .pi./8, .pi./4, 3.pi./8, .pi./2, 5.pi./8, 3.pi./4 and 7.pi./8 radians. These components multiply the received signal; the products generated are low-pass filtered, the filter outputs are then multiplied to generate a phase-correction signal which is applied to a phase correction input of the reference phase source.
These methods, conventionally implemented, are discussed in Digital Communications, by John G. Proakis, 1st Ed., ISBN 0-07-050927-1, pages 193-199.
For all of these systems, the response time of the carrier recovery process is determined by the filtering (or averaging) applied in the generation of the phase correction signal. In previous art, such methods were suitable for and commonly applied to systems where substantial multipath effects did not exist, such as satellite communications links, or fixed terrestrial point-to-point links. Because of the aforementioned fading effects, it is undesirable to perform carrier recovery using methods which do not adapt to variations of the received signal due to fading effects. The signal, during a fade event, is corrupted to some degree by noise energy. Thus, it corrupts the phase estimation of the received signal.
It is also true that, because of the new application of the TDMA method of transmission of information in bursts, it is undesirable not to use the timing of these bursts to time the adjustment of at least the carrier recovery elements of the receiver.
It would be desirable to have a carrier recovery apparatus or method with an adjustable response time to provide the most accurate estimate of the current phase of the received carrier signal. Therefore, there is a need for a carrier recovery method and apparatus having an adjustable response time determined by received carrier signal parameters.